My thesis tackles two related problems that have taken center stage in the recent literature on concepts: • What are the individuation conditions of concepts? Under what conditions is a concept C₁ the same concept as a concept C₂? • What are the possession conditions of concepts? What conditions must be satisfied for a thinker to have a concept C? I will develop a pluralist and contextualist theory of concept individuation and possession: different concepts have different individuation and possession conditions, and contextual factors play a crucial role in determining what concepts we attribute to other subjects when we ascribe propositional attitudes to them. In chapters 1-3, I defend a contextualist, non-Millian theory of propositional attitude ascriptions. Then, I suggest contextualist theories of ascriptions can be applied to the problem of concept individuation/possession. In particular, I use contextualism to provide a new, more effective argument for Fodor's “publicity principle”, according to which concepts must be shared in order for interpersonally applicable psychological generalizations to be possible. Publicity has important implications: in particular, it is inconsistent with existing versions of holism, on which concepts cannot be shared by ordinary thinkers. Nonetheless, in chapters 4-5 I show how holism can still play an important role in our best theory of concepts. More specifically, I argue that the tradition of appealing to modes of presentation in order to give an account of “Frege cases” is in fact committed to holism. To develop a version of holism that will give a successful account of Frege cases without violating publicity, I suggest we should adopt my pluralist-contextualist picture: on that picture, the concepts involved in a Frege case will be holistically individuated and not public, while other concepts will be more coarsely individuated and widely shared. In chapter 6, I will develop this view further by contrasting it with other pluralist theories (Weiskopf) and with rival theories of concepts, such as the localist views defended by Peacocke, Rey and Jackson.